The present invention is directed to an instrument capable of measuring of fluid velocity and more particularly concerns electromagnetic flowmeters.
Electromagnetic flowmeters are well known and are used to measure the volume flow rate of a wide range of fluids. The fluid is usually a liquid, and may be abrasive or non-abrasive, chemically corrosive or passive; the only limitation is that the liquid has some conductivity. Electromagnetic flowmeters offer major advantages over other flow indicating devices, being non-obstructive to the moving fluid and having no moving parts.
The principle of electromagnetic flowmeter operation is based upon Faraday's law: if a conductor moves through a magnetic field, an electrical potential is developed across the conductor in a direction orthogonal to both the conductor and the magnetic field. In the case of an electromagnetic flowmeter, the conductor is the fluid moving through a conduit or a pipe with a magnetic field of more or less parallel flux lines transverse to the fluid flow.
While the principle of electromagnetic flowmeters is relatively simple, the art is the subject of many design refinements. To sense the potential generated across the field diametrically opposed electrodes are placed in contact with the fluid. The electrodes are usually arranged on the periphery of a section of the pipe, orthogonal to both the magnetic field and the direction of the fluid flow. The pipe section may be constructed of nonconducting material whereupon the electrodes may be embedded directly within this material with a surface exposed to the liquid. If the pipe section is constructed of conductive material, the electrodes must be insulated from the pipe.
As the fluid flows through the pipe, it cuts across magnetic lines of flux and develops a potential which can be measured across the electrodes. This electrical potential is a function of both the magnetic strength and the velocity of the fluid. If the magnetic field is held constant, the electrical potential will ideally be a function of the fluid velocity alone. Conductivity of the fluid is not a factor providing it exceeds a minimal value.
The voltage developed across the electrodes is usually amplified by an amplifier known in the art as a "transmitter" or "secondary device". Magnetic flux is preferably supplied by an electromagnet. The flow through the pipe is not perfect so eddy currents may be present at the boundary between the fluid and the pipe. The effect of these currents in the presence of a steady magnetic field is to gradually polarize the electrodes because of electrolytic action so as to provide an erroneous electrical bias. Attempts have been made to make the electrodes less susceptible to polarization under a steady magnetic flux, but most of such methods have proven less than satisfactory.
It, therefore, has become common in the art to alternate the magnetic flux with time so as to prevent polarization of the electrodes. Usually the flux is varied sinusoidally. Unfortunately, other secondary effects may occur because of a time varying magnetic field. A time varying magnetic field will induce voltages on stationary conductors placed within the field, developing voltage between the electrodes that is independent of the fluid velocity. In addition to this electrode voltage or signal, there is also multiple path AC coupling between the magnetic coils and the fluid covered electrodes. These two effects produce signals that combine to produce a signal approximately 90.degree. out of phase with the flow signal, which is called a quadrature signal. The quadrature signal can be minimized by careful design of the sensing electrodes and focusing circuitry but cannot be eliminated completely.
Instead of using the usual sinusoidal current to change the magnetic flux direction, electromagnetic flowmeters driven by square wave currents have been developed such as described by U.S. Pat. No. 3,783,687. With a square wave the time variance is much more instantaneous than the sinusoidal wave so that the quadrature signal is substantially reduced.
One problem with square wave coil current is due to the fact that the coils are inductive and may store one or one and a half joules of energy. Upon discharge a high voltage spike of perhaps 25 kilovolts may be generated. The magnetic drive circuit must be protected from these stresses or electrical arcing may occur. Also arrangements must be made to suppress electrical noise which occurs when the coils are abruptly discharged.
Another serious drawback of a pure square wave is that the the abrupt discontinuity in magnetic flow causes transient voltage signals to appear across the electrodes. U.S. Pat. No. 3,894,430 calls for a clipped sinusoidal coil current. While not a pure square wave, it is possible for transients to be generated at the points where the current is clipped or limited. To avoid the effect of these transients, sample and hold techniques are used to sample the voltages between the electrodes during the interval when the current is constant. The problem is that the transients may not be completely damped at the time of the sample and erroneous voltages can still be generated.
It is desirable to more nearly approximate continuous monitoring. Accordingly, sample rate time must be quite rapid, preventing the luxury of allowing any transients to decay. It is also highly desirable to provide an electromagnetic flowmeter that varies magnetic flux so as to prevent polarization of electrodes and quadrature signals as well as to eliminate transient voltages being induced upon the electrodes thereby allowing a fast sample rate.
Accordingly, an object of the invention is to provide a flowmeter having a minimized quadrature signal while avoiding generation of transients.